Production of optical pulses at a desired wavelength utilizing higher-order-mode (HOM) fiber

ABSTRACT

An apparatus and method for producing optical pulses of a desired wavelength utilizes a section of higher-order-mode (HOM) fiber to receive input optical pulses at a first wavelength, and thereafter produce output optical pulses at the desired wavelength through soliton self-frequency shifting (SSFS) or Cherenkov radiation. The HOM fiber is configured to exhibit a large positive dispersion and effective area at wavelengths less than 1300 nm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Applications60/863,082, filed Oct. 26, 2006, and 60/896,357, filed Mar. 22, 2007,both provisional applications herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to the production of optical pulses at adesired wavelength using higher-order-mode fibers and, more particular,to the utilizing of HOM fiber with a positive dispersion and largeeffective area sufficient to generate high energy, short pulses atwavelengths below 1300 nm, considered useful for numerous applications.

BACKGROUND OF THE INVENTION

Mode-locked femtosecond fiber lasers at 1.03 μm and 1.55 μm have beenimproving significantly in the last several years, particularly withrespect to the achievable output pulse energy (increasing from 1 to ˜10nJ). Even higher pulse energy can be achieved in femtosecond fibersources based on fiber chirped pulse amplification. However, femtosecondfiber sources, including lasers, have seen only limited applications inmultiphoton imaging. The main reason is that they offer very limitedwavelength tunability (tens of nanometer at best), severely restrictingthe applicability of these lasers, making them only suitable for somespecial purposes. In addition, existing femtosecond fiber sources athigh pulse energy (>1 nJ) are not truly “all fiber,” i.e., the output isnot delivered through a single mode optical fiber. Thus, additionalsetup (typically involving free-space optics) must be used to deliverthe pulses to imaging apparatus, partially negating the advantages ofthe fiber source.

Higher-order-mode (HOM) fiber has attracted significant interestrecently, due to the freedom it provides to design unique dispersioncharacteristics in all-solid (i.e., non-“holey”) silica fiber.

The ‘wavelength tunability’ of femtosecond optical sources has beenextensively studied within the phenomenon of soliton self-frequencyshift (SSFS), in which Raman self-pumping continuously transfers energyfrom higher to lower frequencies within an optical fiber. SSFS has beenexploited over the last decade in order to fabricate widelyfrequency-tunable, femtosecond pulse sources with fiber delivery. Sinceanomalous (positive) dispersion (β₂<0 or D>0) is required for thegeneration and maintenance of solitons, early sources that made use ofSSFS for wavelength tuning were restricted to wavelength regimes >1300nm, where conventional silica fibers naturally exhibit positivedispersion.

In addition, Cherenkov radiation has been demonstrated inmicrostructured fibers pumped near their zero-dispersion wavelength. Ingeneral, an ideal soliton requires a perfect balance between dispersionand nonlinearity so that energy becomes endlessly confined to a discretepacket—both spectrally and temporally. When perturbations areintroduced, this stable solution breaks down, allowing the transfer ofenergy between the soliton and the disturbance. Such energy transferoccurs most efficiently in fibers for solitons near the zero-dispersionwavelength. The spectral regime to which energy couples most efficientlyhas been dubbed “Cherenkov radiation” due to an analogous phase matchingcondition in particle physics. The phenomenon of Cherenkov radiation infibers is often associated with SSFS as it allows a convenient mechanismfor more efficient energy transfer between the soliton and the Cherenkovband. In particular, when the third-order dispersion is negative, SSFSwill shift the center frequency of the soliton toward thezero-dispersion wavelength, resulting in efficient energy transfer intothe Cherenkov radiation in the normal dispersion regime. The problem oftunability remains an issue for these arrangements capable of creatingCherenkov radiation.

The recent development of index-guided photonic crystal fibers (PCF) andair-core photonic band-gap fibers (PBGF) have relaxed this tunabilityrequirement somewhat, with the ability to design large positivewaveguide dispersion and therefore large positive net dispersion inoptical fibers at nearly any desired wavelength. This development hasallowed for a number of demonstrations of tunable SSFS sourcessupporting input wavelengths as low as 800 nm in the anomalousdispersion regime.

Unfortunately, the pulse energy required to support stable Raman-shiftedsolitons below 1300 nm in index-guided PCFs and air-core PBGFs is eitheron the very low side, a fraction of a nJ for silica-core PCFs, or on thevery high side, greater than 100 nJ (requiring an input from anamplified optical system) for air-core PBGFs. The low-energy limit isdue to high nonlinearity in the PCF. In order to generate large positivewaveguide dispersion to overcome the negative dispersion of thematerial, the effective area of the fiber core must be reduced. Forpositive total dispersion at wavelengths less than 1300 nm, thiscorresponds to an effective area, A_(eff) of 2-5 μm², approximately anorder of magnitude less than conventional single mode fiber (SMF). Thehigh-energy limit is due to low nonlinearity in the air-core PBGF wherethe nonlinear index, n₂, of air is roughly 1000 times less than that ofsilica. In fact, most microstructure fibers and tapered fibers withpositive dispersion are intentionally designed to demonstrate nonlinearoptical effects at the lowest possible pulse energy, while air-corePBGFs are often used for applications that require linear propagation,such as pulse delivery.

For these reasons, previous work using SSFS below 1300 nm was performedat soliton energies either too low or too high (by at least an order ofmagnitude) for many practical applications, such as multiphoton imaging,where bulk solid state lasers are currently the mainstay for theexcitation source.

The present invention is directed to overcoming these and otherdeficiencies in the state of the art.

SUMMARY OF THE INVENTION

The present invention relates to a higher-order-mode (HOM) fiber moduleoperable to generate energetic, short output pulses of light atwavelengths amenable to various applications, while also providing adegree of wavelength tunability. In particular, the inventive HOM moduleincludes a section of HOM fiber with anomalous (positive) dispersion anda large effective area, characteristics that create a solitonself-frequency shift sufficient to move an incoming stream of pulses atone wavelength to a stream of pulses at a second, desired wavelengthassociated with a specific application. These dispersion characteristicshave also been found to allow for the creation of soliton Cherenkovradiation at wavelengths below 1300 nm, with usable energy in the rangeof 1-10 nJ.

Additionally, the HOM fiber module of the present invention provides theability to compensate the dispersion of an optical pulse that is chirpedat its input. Therefore, the HOM module provides a sufficient amount ofdispersion to provide a transform-limited pulse at a predeterminedlocation within the HOM fiber such that the pulse undergoes frequencyshift by either of the SSFS or Cherenkov effects described above.

In accordance with the present invention, the HOM module comprises aninput mode converter (for converting from the conventional LP₀₁ mode toa higher-order mode), a section of HOM fiber coupled to the input modeconverter for generating the desired self-frequency shift to a desiredoutput wavelength, and (when necessary) an output mode converter (forconverting the wavelength-shifted pulses back to the conventional LP₀₁mode or any other desired spatial profile).

In one embodiment, in-fiber long period gratings (LPGs) are used for theinput and output mode converters, thus minimizing the amount of opticalloss present at the junction between the mode converters and the HOMfiber.

The HOM fiber portion of the module is configured in one embodiment toinclude a wide, low index ring cladding area, separated from a highindex core region by a trench. The index values and dimensions of thering, trench and core are selected to provide the desired amount ofanomalous dispersion and the size of the effective area. One set ofacceptable values for use in accordance with the present invention is adispersion on the order of +60 ps/nm-km and an effective area ofapproximately 44 μm². Another set of acceptable values are defined bythe wavelength range within which the dispersion is anomalous(positive), this range being between 10 and 300 nm. Yet another set ofacceptable values are defined by the maximum achievable dispersion inthe wavelength range of interest, this value ranging from 0 to +3000ps/nm-km. With respect to the effective area, acceptable values ofA_(eff) for the purposes of the present invention range from about 5 to4000 μm².

The present invention also relates to a method of producing outputoptical pulses having a desired wavelength. The method includesgenerating input optical pulses and delivering the input pulses to anHOM fiber module to alter the wavelength of the input optical pulsesfrom the first wavelength to the second, desired wavelength by solitonself-frequency shifting (SSFS) within the HOM fiber module.

In one embodiment, the method can further include converting the spatialmode of the input signal into a higher-order mode at the input of theHOM fiber module, and thereafter reconverting the output of the HOMfiber module back to the original spatial mode or to any other desiredmode profile.

It is an advantage of the present invention that the HOM module iscapable of achieving these characteristics at wavelengths below 1300 nm,heretofore not accomplished in an all-silica (non-holey) fiber.

Further, the HOM module of the present invention is designed such thatthe difference between the effective index n_(eff) of the mode in whichsignal propagation is desired is separated from that of any other guidedmode of the fiber by greater than 10⁻⁵, thus providing for enhancedmodal stability of the signal.

In one embodiment, the input comprises a single mode fiber (SMF) splicedto the HOM fiber before mode conversion, with the properties of thesplice ensuring that signal propagation in the HOM fiber occurspredominantly in the LP₀₁ mode, further enhancing modal stability forthe signal.

Other and further aspects and embodiments of the present invention willbecome apparent during the course of the following discussion and byreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a comparison of modal behavior between conventional LP₀₁(single mode fiber, top—schematic) and LP₀₂ (bottom—simulated) modes.FIG. 1A: Near-field images. FIG. 1B: Mode profiles at variouswavelengths. Conventional mode transitions from high to low index;designed HOM shows opposite evolution. Grey background denotes indexprofile of the fiber. FIG. 1C: Resultant total dispersion (D_(total),solid). Also shown are silica material dispersion (D_(m), dashed) andzero-dispersion line (dotted). Arrows show contribution of waveguidedispersion (D_(w)) to total dispersion.

FIG. 2 is an index profile of the HOM fiber.

FIG. 3 shows an experimentally measured near-field image LP₀₂ mode withan effective area A_(eff)˜44 μm².

FIG. 4 is a refractive index profile for an HOM fiber of the presentinvention, indicating the set of six different parameters than may beadjusted to provide the desired positive dispersion and large effectivearea;

FIG. 5 is a graph of the simulated total dispersion vs. wavelengthcurves for a variety of profiles, forming by adjusting one or more ofthe parameters shown in FIG. 4;

FIG. 6 shows index vs. radial position of the designed and fabricatedfiber measured at several perform positions;

FIG. 7 illustrates an exemplary HOM module for converting an inputwavelength to a desired output wavelength in accordance with the presentinvention; and

FIG. 8 is a graph of the transmission bandwidth of the HOM module ofFIG. 7.

DETAILED DESCRIPTION

The present invention is directed to an arrangement for producing highenergy, femtosecond output light pulses over a tunable wavelength rangefor wavelengths less than 1300 nm, using a relatively new type offiber—higher-order-mode (HOM) fiber—that yields strong anomalousdispersion in the output wavelength range. Advantageously, the HOM fiberis an all-solid silica fiber structure (i.e., does not include air gapsor other microstructures) where the guidance mechanism is conventionalindex guiding. This represents a major breakthrough in fiber design,inasmuch as it was not previously considered possible to obtainanomalous dispersion at wavelengths shorter than 1300 nm in anall-silica optical fiber.

In accordance with the present invention, a higher-order-mode (HOM)fiber has been developed that is capable of achieving a strong positive(anomalous) waveguide dispersion (D_(w)) for the LP₀₂ mode atwavelengths less than 1300 nm. In particular, an HOM fiber has beenformed that exhibits +60 ps/km-nm dispersion for the LP₀₂ mode in the1060-nm wavelength range. Combined with in-fiber gratings, this resulthas enabled the construction of an HOM anomalous dispersion element(hereinafter referred to as an “HOM module”) with low loss (˜1%), and aneffective area A_(eff) (e.g., ˜44 μm²) that is ten times larger thanconventional photonic crystal fibers (PCFs). Significantly, the guidancemechanism is index-guiding, as in standard fibers. Therefore, theinventive HOM fiber retains the desirable properties of such fibers,including low loss, bend resistance, and lengthwise invariance (in termsof loss, dispersion, etc.), making such a fiber attractive for a varietyof applications. By utilizing the phenomenon of SSFS, for example, aninput optical signal at a first, input wavelength can be shifted to asecond, output wavelength after propagating through the HOM fiber of thepresent invention. Additionally, an HOM fiber module in accordance withthe present invention can be used as a femtosecond fiber source at 1300nm using soliton Cherenkov radiation in the HOM fiber to efficientlyconverter a 1030 nm femtosecond fiber source to the desired 1300 nmwavelength.

FIG. 1 provides an intuitive picture for the dispersive behavior of theguided modes by comparing the properties of the LP₀₁ mode typicallyassociated with convention single mode fiber, and the LP₀₂ mode assupported within the inventive HOM fiber. In particular, FIG. 1( a)shows modal images for the fundamental LP₀₁ mode (top) and the higherorder LP₀₂ mode (bottom). FIG. 1( b) shows the evolution of these modeprofiles as a function of wavelength, in particular at 800 nm, 1040 nmand 1250 nm. The gray background in FIG. 1( b) is used to illustrate therefractive index profile of the fiber. As shown in the top set of modalimages, the LP₀₁ mode monotonically transitions from the high indexcentral core to the surrounding lower index regions as the wavelengthincreases from 800 nm to 1040 nm, and finally to 1250 nm. Thus, thefraction of power traveling in lower index regions increases withincreasing wavelength. Since the velocity of light increases as therefractive index of the medium drops, the LP₀₁ mode experiences smallergroup delays as wavelength increases.

Waveguide dispersion (D_(w)), which is the derivative of group delaywith respect to wavelength, is thus negative for the LP₀₁ mode.Therefore, in wavelength ranges in which material dispersion (D_(m)) isitself negative, the conventional LP₀₁ mode can achieve only negativetotal dispersion values, where “total dispersion” D_(total) is definedas the sum of waveguide dispersion and material dispersion. This isillustrated in FIG. 1( c) (top), which plots material dispersion D_(m)as well as total dispersion D_(total) of the LP₀₁ mode in the 1060-nmwavelength range.

In contrast and in accordance with the present invention, thehigher-order LP₀₂ mode is designed to have the mode evolution shown inFIG. 1( b) (bottom). Again, the gray background is used to illustratethe refractive index profile for the fiber supporting this mode. Asshown, when the wavelength increases from 800 nm to 1040 nm, and then to1250 nm, the mode evolves in the opposite direction as the conventionalfiber described above. That is, with reference to the diagrams along thebottom of FIG. 1( b), the mode transitions from the lower index regionsto the higher index core as the wavelength increases from 800 nm to 1250nm. The LP₀₂ mode thus experiences larger group delays as the wavelengthincreases.

Therefore, the LP₀₂ mode will exhibit a wavelength dispersion D_(w) thatis positive over this entire range as the mode transitions from thecladding to the core. This is illustrated in FIG. 1( c) (bottom), whichshows the wavelength range where this transition occurs. Indeed, verylarge positive values of D_(w) may be obtained, vastly exceeding themagnitude of the material dispersion D_(m) (which, as mentioned above,is negative over the same range). As a result of the substantialdifference in magnitude between the waveguide dispersion and thematerial dispersion, the LP₀₂ mode that propagates along an HOM fiberwill exhibit a total dispersion D_(total) that is positive (anomalousdispersion).

It is to be noted that this evolution is governed by the “attractive”potential of various high index regions of the waveguide, and can thusbe modified to achieve a variety of dispersion magnitudes, slopes andbandwidths. This yields a generalized recipe to obtain positivedispersion in a variety of wavelength ranges.

FIG. 2 shows the index profile of an exemplary HOM fiber formed inaccordance with the present invention to provide this positivedispersion value, where a broad, low index ring 10 serves tosubstantially guide the LP₀₂ mode at shorter wavelengths. As describedabove, the mode will then transition to a small, high index core 12 aswavelength increases (as described above in associated with FIG. 1( b),bottom). The experimentally recorded near-field image of this LP₀₂ modeis shown in FIG. 3, where measurements have shown that this exemplaryHOM fiber will exhibit an effective area A_(eff) of approximately 44 μm²at 1080 nm.

The well-known physics of SSFS dictates that the wavelength tuning rangeis limited by the range within which the dispersion of the fiber mode isanomalous (positive). In other words, for a tuning range of λ_(tuning),the dispersion-zero crossings of the dispersion curves must also beseparated by at least the same amount λ_(tuning). For many applications,it is desirable that this range be at least 300 nm. More broadly, atuning range anywhere between 10 nm and 2000 nm may be considereduseful. In general, the range of such tuning, and correspondingly theenergy carried by the shifted soliton, scale with D*A_(eff) for thewavelength and the mode in which the soliton signal resides.

The well-known physics of generation of Cherenkov radiation, on theother hand, requires the existence of a zero-dispersion crossing. If anoptical soliton exists in its vicinity in the anomalous dispersionwavelength range, then Cherenkov radiation is generated in the spectralregion on the other side of this zero-dispersion wavelength—i.e., in theregion where the dispersion is normal. The exact spectral location ofthe generated wave is further governed by the dispersion slope of thefiber mode. Again, the energy of the converted radiation scales asD*A_(eff) for the mode in which the optical radiation resides.

In accordance with the present invention, therefore, the fiber designproblem reduces to one of configuring an HOM fiber with the requiredvalue of D*A_(eff) at the output wavelengths of the dispersion curve.The general fiber index profile for achieving D_(w)>0 for the LP₀₂ modeis shown in FIG. 4. While FIG. 1 provides the physical intuition forD_(w)>0 in an HOM fiber, achieving target dispersion D and effectivearea A_(eff) values requires a numerical optimization of the sixparameters shown in FIG. 4, namely, the indices and dimensions of ring10, trench 14 and core 12. There are two ways to achieve a largedispersion (D) value; one is by increasing refractive index valuesΔN_(core) and ΔN_(ring), but this may be at the expense of the effectivearea A_(eff). The second approach is by increasing r_(ring) as well asr_(trench). Increasing r_(ring) will enhance the mode size, whileincreasing r_(trench) will provide for greater effective index changesas the mode transitions, resulting in larger dispersion.

The key to achieving the desired properties is a mode that cantransition (as a function of wavelength) through well-defined, sharpindex steps in the fiber's index profile. Therefore, the fabricationprocess must be capable of producing both large index steps as well assteep index gradients, as shown in FIG. 4. The ideal means to achievethis is the Modified Chemical Vapor Deposition (MCVD) process, whichaffords the best layer-to-layer control of refractive index of allestablished fabrication technologies for fibers.

Dimensional scaling of the preform can also be used to shift thewaveguide dispersion D_(w) in order to achieve the D*A_(eff) necessaryfor the desired output wavelength ranges. This is known in the art ofoptical waveguides as complementary scaling, which states thatwavelength and dimension play a complementary role in the wave equationand, therefore, are interchangeable. However, it is to be noted thatthis is true only for the waveguide component of dispersion, D_(w).Changes in the material dispersion, D_(m), are not complementary and, asa result, the total dispersion D is not wavelength scalable. In otherwords, to move the dispersion curve that provides satisfactory operationin the 1030 nm wavelength range to the 775 nm spectral range, thedispersion D_(w) needs to be high enough to counteract the strongnegative trend for D_(m) as wavelength decreases. Therefore, achievingsimilar properties at lower wavelengths needs the use of bothdimensional scaling and the above-described dispersion-increasingconfigurations.

To achieve the higher 5- to 10-nJ output pulse energies, the design ofan inventive HOM in this range requires a D*A_(eff) value that is fiveto ten times greater than that associated with providing output pulsesin the 1-2 nJ range. The main difficulty is to simultaneously achievethe large values of D*A_(eff) while maintaining λ_(tuning) atapproximately 300 nm. FIG. 5 illustrates the simulated total D vs.wavelength curves for a variety of acceptable profiles, where thematerial dispersion value of silica, D_(m), is also shown. An importantconstraint applied in generating the profiles shown in FIG. 5 is thatthe effective index n_(eff) of the HOM in which signal propagation isdesired (i.e., the mode for which the dispersion curves are shown), isvastly separated from the n_(eff) of any other mode that may be guidedin the fiber. The large separation in n_(eff) between modes ensures thatthe signal that is introduced in the HOM predominantly propagates onlyin that mode and does not randomly coupled to any other mode. Suchrandom coupling may occur due to bends and other environmentalperturbations, and typically the n_(eff) difference between the modesshould be greater than 10⁻⁵ to avoid this type of coupling.

FIG. 6 shows an example of the designed and fabricated index profilesfor an HOM fiber formed in accordance with the present invention thatyields a large positive dispersion in the 1060-nm wavelength range. Thepreform profiles closely match the design profile in both index valuesand the steep index gradients. Also shown in FIG. 6 are index profilesfrom different sections of the preform, showing the uniformity of theMCVD process in fabrication an HOM fiber whose properties are invariantas a function of fiber length. This robust fiber fabrication process iscritical to provide a constant zero-dispersion wavelength in an HOMfiber for SSFS, and is a significant advantage of the inventive HOMfiber over the prior art bandgap fibers.

FIG. 7 illustrates an exemplary wavelength converting HOM module 20formed in accordance with the present invention, including a section ofHOM fiber 22 having the characteristics as described above inassociation with the above Figures to provide high energy femtosecondpulses at wavelengths less than 1300 nm. In accordance with the presentinvention, HOM module 20 utilizes SSFS, or a combination of SSFS withCherenkov radiation, to shift the wavelength of an incoming signal to anoutput wavelength selected for a specific application (the outputwavelength less than 1300 nm).

Further, in accordance with the present invention, HOM module 20provides for dispersion compensation prior to wavelength shifting, suchthat chirped incoming pulses are “de-chirped” with the required amountof dispersion within HOM fiber 22. Thereafter, the de-chirped pulsesundergo SSFS and/or Cherenkov radiation to generate the output pulses atthe desired wavelength.

For proper operation of HOM module 20, an input mode converter 24 isneeded to convert an incoming Gaussian-shaped LP₀₁ mode signal into thedesired LP₀₂ mode. One preferred method for providing the modeconversion is with one or more in-fiber long period gratings (LPGs).This type of grating can be permanently formed in fibers bylithographically transferring a grating pattern from an amplitude maskto the fiber using a UV laser. For efficient grating formation, thefiber is typically saturated with deuterium, which acts as a catalystfor the process, resulting in UV-induced index changes in thegermanosilicate glass. In another embodiment, the input mode convertermay convert any arbitrary incoming spatial profile of light into the HOMthat is desired to be propagated in the HOM fiber. For someapplications, an output mode converter may be used to transform thehigher-order-mode into another spatial mode. In the illustration of FIG.7, an output mode converter 26 is shown as disposed at the output of HOMfiber 22 to transition the wavelength-shifted LP₀₂ mode signal back intoa conventional LP₀₁ signal. More generally, an output mode converter canbe used to convert the HOM into any desired spatial profile of light.

Alternatively, in some applications, it may be desired that no outputmode converter is used, inasmuch as the wavelength-shifted radiationalready exhibits the desired spatial mode profile. In these cases,therefore, the need for an output mode converted is obviated. In yetanother embodiment, the HOM module may comprise a plurality of separateHOM fiber sections coupled together in series, using fiber splicingtechniques or another mode converter to join together the adjacentsections. If they are joined by splices, the HOM in the first fiber isexpected to adiabatically transition to the same mode order in thesecond fiber. If they are joined together by means of a mode converter,on the other hand, the mode order from the first fiber to the secondfiber can also be changed. Such arrangements may be desired inapplications where, in order to increase the λ_(tuning) for SSFS, twoconcatenated sections will provide a much larger tuning range than thatassociated within only a single HOM fiber section. Alternatively, sucharrangements may allow for changing the dispersion slope of thezero-dispersion crossing, as may be required for adjusting thewavelength at which Cherenkov radiation occurs. In the case where a modeconverter is used to join two sections of HOM fiber, it is known fromthe prior art that such mode converters may be tunable, with thecapability of switching light from one incoming HOM fiber to any of aset of outgoing HOM fibers (including, of course, reflecting back intothe incoming HOM fiber). If a tunable mode converted is employed in thiscase, the resulting HOM module will additional provide a means todynamically change the effective optical path length of the fiber and,by extension, its dispersion, dispersion-zero and/or dispersion slope(as may be desired for different SSFS and Cherenkov applications). Thus,a module with adjustable HOM fiber lengths may be designed and isconsidered to fall within the scope of the present invention. Indeed, inembodiments that utilize a multiple number of concatenated HOM fibersections, tunable mode converters may be used at the interface betweenany two sections.

LPGs offer coupling between co-propagating modes of a fiber and havefound a variety of applications as spectral shaping elements andmode-conversion devices. However, LPGs are normally narrow-band devices,and while they offer strong mode coupling (>99%), the spectral width ofsuch coupling was typically limited to a range of 0.5 to 2 nm, toonarrow for a femtosecond pulse. To overcome the spectral limitation,reports have shown that the LPG bandwidth can be extended to greaterthan 60 nm in some cases, if the fiber waveguide is configured to yieldtwo modes with identical group velocities. It is to be noted that thelarge bandwidth of HOM module 20, as shown in FIG. 8 (i.e.,approximately 51 nm), is uniquely enabled by the dispersive design ofthe fiber, which enables matching the group velocities of the twocoupled modes. It is a significant aspect of the present invention thatthe utilization of the LPGs allows for the formation of an “all-fiber”tunable femtosecond pulse source.

Referring again to FIG. 7, HOM module 20 is spliced to an input singlemode fiber 30 at input long period grating 24. The splice between singlemode fiber 30 and input LPG 24 is configured such that the signalpredominantly resides in the LP₀₁ mode, thus ensuring mode conversionwith high efficiency and also minimizing signal propagating in any modeother than the desired mode. This enables a device to be constructed inwhich the signal experiences high modal stability, even in the presenceof bends and other environmental perturbations. Indeed, input singlemode fiber 30 may be the output fiber of a laser source (not shown),avoiding any spurious mode coupling, especially in systems where thechirped output of the laser source needs to be directly coupled into theHOM fiber module.

As mentioned above, output long period grating 26 is used to convert thebeam back to a Gaussian output. Dispersion-matching configurations arepreferably used that yield ultra-large bandwidths, ensuring that theoutput pulse is always converted back to a Gaussian profile, within atuning range of approximately 250 nm. An important consideration foroutput grating 26 is its length. Since the energetic output pulses aresolitons for the specific combination of dispersion D and effective areaA_(eff) of the LP₀₂ mode, nonlinear distortions may occur when thesignal converts to the LP₀₁ mode (having a smaller A_(eff)) at theoutput. However, the length over which the signal travels in the LP₀₁mode, and hence the distortion it accumulates, can be minimized. Thehigh index core of HOM fiber 22 enables the use of an output long periodgrating 26 of lengths less than 5 mm, which implies that light residesin the LP₀₁ mode for less than 2.5 mm and therefore largely avoidsnonlinear distortions. It is to be noted that the requirement for“short” LPGs actually complements the need for broad bandwidthoperation, since the conversion bandwidth is typically inverselyproportional to the grating length.

Although preferred embodiments have been depicted and described indetail herein, it will be apparent to those skilled in the relevant artthat various modifications, additions, substitutions and the like can bemade without departing from the spirit of the invention and these aretherefore considered to be within the scope of the invention as definedin the claims which follow.

1. An apparatus for providing optical output pulses of a desired outputwavelength from an input optical signal operating at a differentwavelength, the apparatus comprising: an input mode converter forreceiving the input signal and converting the spatial mode of the inputsignal into a higher-order-mode (HOM) signal; and a section ofhigher-order-mode (HOM) fiber coupled to the output of the input modeconverter for receiving the input optical signal and thereafter produceas an output an optical signal at the desired output wavelength, whereinthe section of HOM fiber is configured to exhibit a positive dispersionand large effective area sufficient to produce optical output pulses atthe desired output wavelength, the positive dispersion also sufficientto perform dispersion compensation on the input optical signal, reducingthe presence of chirp prior to producing the optical output signal.